Transparent carbon nanotube electrode using conductive dispersant and production method thereof

ABSTRACT

Disclosed is a transparent carbon nanotube (CNT) electrode using a conductive dispersant. The transparent CNT electrode comprises a transparent substrate and a CNT thin film formed on a surface the transparent substrate wherein the CNT thin film is formed of a CNT composition comprising CNTs and a doped dispersant. Further disclosed is a method for producing the transparent CNT electrode. 
     The transparent CNT electrode exhibits excellent conductive properties, can be produced in an economical and simple manner by a room temperature wet process, and can be applied to flexible displays. The transparent CNT electrode can be used to fabricate a variety of devices, including image sensors, solar cells, liquid crystal displays, organic electroluminescence (EL) displays and touch screen panels, that are required to have both light transmission properties and conductive properties.

This non-provisional application claims priority to Korean PatentApplication No. 10-2006-0100726 filed on Oct. 17, 2006, and all thebenefits accruing therefrom under 35 U.S.C. §119(a), the content ofwhich is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transparent carbon nanotube (“CNT”)electrode comprising a conductive dispersant and a method for producingthe same. More specifically, the present invention relates to atransparent CNT electrode comprising a transparent substrate and a CNTthin film formed on a surface of the transparent substrate wherein theCNT thin film contains carbon nanotubes (“CNTs”) and a doped dispersant,and a method for producing the transparent CNT electrode. Thetransparent CNT electrode of the present invention can be produced by awet process, has excellent conductive properties, and can be applied toflexible displays.

2. Description of the Related Art

Transparent electrodes having a transparent substrate and a conductivefilm formed on the transparent substrate are now widely used tofabricate a variety of devices, including image sensors, solar cells,liquid crystal displays, organic electroluminescence (“EL”) displays andtouch screen panels, that are required to have both light transmissionproperties and conductive properties.

Indium tin oxide (“ITO”) electrodes have been predominantly used aselectrodes for transparent substrates because ITO shows a strongtendency to form a thin film on glass substrates and exhibits excellentlight transmission properties and conductive properties. Vacuumdeposition equipment is used to produce ITO electrodes. Particularly,sputtering equipment can be used in view of the excellentcharacteristics conveyed to the film by the sputtering technique.However, production of transparent electrodes by sputtering techniquesrequires a high processing temperature of 200° C. or higher, sometimes400° C. or higher. Therefore, sputtering techniques are not suitable forthe production of flexible displays that require transparent electrodes.Moreover, the use of the inflexible ITO electrodes in flexible displayscauses increased sheet resistance and poor durability which isproblematic for flexible displays.

To address these problems, extensive research is actively underway toinvestigate use of transparent electrodes based on carbon nanotubes(CNTs) as materials for conductive films formed on transparentsubstrates.

Carbon nanotubes (CNTs) are tubular materials made of carbon atoms inwhich one carbon atom is bonded to other adjacent carbon atoms in theform of a hexagonal-based honeycomb structure. CNTs are highlyanisotropic, have various structures, such as single-walled,double-walled, multi-walled and rope (i.e., helical) structures, andhave an extremely small diameters in the nanometer (1×10⁻⁹ meter) range.CNTs are known to have excellent mechanical properties, good electricalselectivity, superior field emission properties, highly efficienthydrogen storage properties, and the like. Particularly, CNTs can beadvantageously used to form electrically conductive films due to theirhigh electrical conductivity. CNTs can be synthesized by known methodsincluding electrical discharge, pyrolysis, laser deposition, plasmachemical vapor deposition, thermal chemical vapor deposition, orelectrolysis.

CNTs must be dispersed in suitable dispersion media in order to formconductive films. However, CNTs tend to aggregate by surface attraction,in particular by the intermolecular force referred to as Van der Waalsattraction, where CNT's have a Van der Waals attraction of about 950meV/nm. Since such aggregation of CNTs impedes the formation ofthree-dimensional networks that are capable of improving the mechanicalstrength and conductive properties of the CNTs, it is necessary todisperse the CNTs in suitable dispersion media.

Since most organic dispersants act as insulators, CNT thin films formedusing organic dispersants generally exhibit poor conductive properties.Various efforts have been made to remove residual organic materials,which can act as insulators, from CNT films. For example, after adispersion of CNTs and an organic material in water is used to form aCNT film, the organic material is removed from the film by dipping thefilm in water (Nano letters 2005, Vol. 5, No. 4, pp. 757-760). However,this method has difficulty in completely removing the organic materialfrom the CNT film and ensuring reproducibility.

Further, although CNTs may be sufficiently dispersed in the conductivedispersant to form a conductive film, deterioration in the conductivityof the conductive film is inevitable because the conductive dispersantsurrounds the surface of the CNTs and the conductivity of the conductivedispersant is much lower than that of the CNTs.

BRIEF SUMMARY OF THE INVENTION

Therefore, in view of the problems of the prior art, the presentinvention provides a transparent CNT electrode that can be applied toflexible displays and has increased conductivity.

In another aspect of the present invention a method is provided forproducing the transparent CNT electrode.

In an embodiment, there is provided a transparent CNT electrodecomprising a transparent substrate and a CNT thin film formed on asurface of the transparent substrate wherein the CNT thin film is formedof a CNT composition comprising CNTs and a doped dispersant.

In another embodiment, the substrate can be a transparent inorganicsubstrate, including a glass or quartz substrate, or a flexibletransparent substrate made of a material selected from the groupconsisting of polyethylene terephthalate, polyethylene naphthalate,polyethylene sulfone, polycarbonate, polystyrene, polypropylene,polyester, polyimide, polyetheretherketone, polyetherimide, acrylicresins, olefin-maleimide copolymers and norbornene-based resins.

In a further embodiment of the present invention, the CNTs used in theCNT composition are selected from the group consisting of single-walledcarbon nanotubes, double-walled carbon nanotubes, multi-walled carbonnanotubes, rope carbon nanotubes, and combinations thereof.

In another embodiment of the present invention, the doped dispersant canbe a conductive dispersant. The conductive dispersant can have astructure that includes a head containing an aromatic ring with a highaffinity for the CNTs and one tail or two tails that are the same ordifferent, and with an affinity for a dispersion medium.

In an embodiment, a method for producing a transparent CNT electrodecomprises (a) preparing a CNT composition comprising a CNT and aconductive dispersant, (b) forming a CNT thin film containing aconductive dispersant on a surface of a transparent substrate with theCNT composition, and (c) doping the conductive dispersant contained inthe CNT thin film.

In another embodiment, step (c) can include the sub-steps of i) dopingthe CNT thin film formed on the transparent substrate in a dopantsolution, ii) washing excess dopant remaining in the CNT thin film, andiii) drying the doped CNT thin film.

In an alternative embodiment of the present invention, step (c) can becarried out with a vapor of iodine, bromine, chlorine, iodinemonochloride, iodine trichloride, or iodine monobromide.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and other advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view of an exemplary transparentCNT electrode according to an embodiment;

FIG. 2 is a scanning electron micrograph (“SEM”) showing a surface of anexemplary transparent CNT electrode produced in Example 1;

FIG. 3 a is a photograph showing the state in which CNT thin films aredipped in different dopant solutions during doping of the CNT thin filmsto produce the exemplary transparent CNT electrodes in Examples 1 and 2of the present invention, and FIG. 3 b is a photograph showing the statein which CNT thin films are washed during doping of the CNT thin filmsto produce the exemplary transparent CNT electrodes in Examples 1 and 2;

FIG. 4 is a graph showing doping effects for the exemplary transparentCNT electrodes produced in Example 3 according to the concentration ofdopant solutions and doping time; and

FIG. 5 is a graph showing doping effects for the exemplary transparentCNT electrodes produced in Example 4 with varying light transmittancevalues of the transparent CNT electrodes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in greater detail withreference to the accompanying drawings.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “disposed on” or “formed on” another element, theelements are understood to be in at least partial contact with eachother, unless otherwise specified.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The use of the terms “first”, “second”, and the like do notimply any particular order but are included to identify individualelements. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

In the drawings, like reference numerals in the drawings denote likeelements and the thicknesses of layers and regions are exaggerated forclarity.

In one aspect, a transparent CNT electrode comprises a transparentsubstrate and a CNT thin film formed on the transparent substrate. TheCNT thin film contains carbon nanotubes (CNTs) and a doped dispersant.The use of the doped dispersant, particularly a doped conductivedispersant, in the transparent CNT electrode facilitates migration ofcharges between the CNTs to achieve excellent conductive properties ofthe transparent CNT electrode without any deterioration in lighttransmittance. In addition, since the transparent CNT electrode of thepresent invention can be produced by wet process at room temperature, itcan be applied to flexible substrates. As a result, the transparent CNTelectrode is useful in a wide range of applications.

FIG. 1 is a schematic cross-sectional view of a transparent CNTelectrode according to an embodiment.

As shown in FIG. 1, the transparent CNT electrode of the presentinvention comprises a transparent substrate 10 and a CNT thin film 20disposed on a surface of transparent substrate 10, wherein the CNT thinfilm contains CNTs 21 and a doped dispersant 22.

The transparent substrate 10 used in the transparent CNT electrode canbe of any type so long as it is transparent, specific examples of whichinclude transparent inorganic substrates, such as glass and quartzsubstrates, and flexible transparent substrates, such as plasticsubstrates. Examples of suitable materials for the flexible transparentsubstrates include polyethylene terephthalate, polyethylene naphthalate,polyethylene sulfone, polycarbonate, polystyrene, polypropylene,polyester, polyimide, polyetheretherketone, polyetherimide, acrylicresins, olefin-maleimide copolymers, and norbornene resins. Thesematerials can be used either alone or in a combination thereof.

The CNTs 21 used in the CNT composition constituting the CNT thin filmis not particularly restricted so long as the advantages of the presentinvention are not impaired. Specifically, the CNTs 21 can be selectedfrom the group consisting of single-walled carbon nanotubes,double-walled carbon nanotubes, multi-walled carbon nanotubes, ropecarbon nanotubes, and combinations thereof. Where single-walled carbonnanotubes are desired for use, metallic carbon nanotubes can beselectively separated by a chemical separation process before use.

The dispersant used to disperse the CNTs is not particularly restricted.In an embodiment, the dispersant can be a conductive dispersant. Theconductive dispersant can have a structure that consists of a headcontaining an aromatic ring with a high affinity for the CNTs and onetail or two tails that are the same or different, and with an affinityfor the dispersion medium.

More specifically, the conductive dispersant may be a compound having astructure in which a head selected from groups represented by Formula 1is substituted with one tail or two tails that are the same ordifferent, and selected from groups represented by Formula 2:

[Ar—(X)_(a)]_(l)   (1)

wherein Ar is a C₆ aromatic group or a C₄-C₅ heteroaromatic groupcontaining at least one heteroatom selected from sulfur (S), nitrogen(N) or oxygen (O),

X is NH— or CH═CH—,

a is 0 or 1, and

l is an integer from 5 to 60;

wherein Y is selected from the group consisting of substituted andunsubstituted C₁-C₁₀ alkylene groups, substituted and unsubstitutedC₁-C₁₀ alkenylene groups, substituted and unsubstituted C₁-C₁₀alkynylene groups, and substituted and unsubstituted C₆-C₂₀ arylalkylenegroups,

Z is selected from the group consisting of —H, —CH₃, —OH, carboxylicacid and its salts, sulfonic acid and its salts, and phosphoric acid andits salts,

b is 0 or 1,

m is an integer from 1 to 9, and

n is an integer from 0 to 9.

The conductive dispersant can also be poly(3,4-ethylenedioxythiophene).In an embodiment, the head of the dispersant is polythiophene.

The dispersant exists in a doped state. Generally, the surface of theCNTs contained in the CNT thin film is surrounded by the dispersant.Since the dispersant has a lower conductivity than the CNTs, itfunctions as an insulator between the CNTs, resulting in a decrease inthe total conductivity of the CNT thin film.

The use of the doped dispersant in the transparent CNT electrodefacilitates migration of the CNTs to achieve improved conductiveproperties of the CNT thin film. The mechanism responsible for theimprovement in conductive properties will be explained in greater detailbelow.

The conductive dispersant has the same composition as a conductivepolymer. Depending on the dispersion medium and the CNTs used, themolecular weight of the conductive dispersant is controlled and one ortwo functional groups (which correspond to the tails of the dispersant)are introduced into the conductive dispersant. The conductive dispersanthas basic characteristics similar to those of a conductive polymer. Thatis, a conductive polymer has a conjugated structure in which singlebonds and double bonds are alternately repeated, and generally exhibitsinsulator (or semiconductor) properties. However, when a conductivepolymer is chemically treated (i.e., is doped), its properties changedfrom insulator (or semiconductor) properties to conductor (orsemiconductor) properties. This chemical treatment (i.e., doping) allowseach bond between successive carbon atoms of the conductive polymer tohave about one and a half bonds (i.e., a bond order of about 1.5)instead of single bonds (a bond order of 1) and double bonds (a bondorder of 2) alternately repeated in the conjugated structure, so thatthe electron density of the conductive polymer is delocalized, therebyfacilitating the intramolecular and intermolecular migration ofelectrons. For example, the following Structural Formula 1 shows thestates of polarons and bipolarons acting as charge carriers when apolythiophene polymer is doped.

The type of dopant performing the above functions in the transparent CNTelectrode of the present invention is not especially restricted. As thedopant, a p-type dopant (an electron acceptor) or an n-type dopant (anelectron acceptor) can be used.

Specific examples of suitable p-type dopants that can be used include,but are not limited to, Lewis acids such as for example PF₅, AsF₅, SbF₅,ClO₄ ⁻, BF₄ ⁻, BF₃, BCl₃, BBr₃, SO₃, NO₂(SbF₆), NO(SbCl₆), or NO₂(BF₄);protic acids such as for example H₂SO₄, HClO₄, HNO₃, FSO₃H, or CF₃SO₃H;transition metal halides such as for example FeCl₃, MoCl₅, WCl₅, SnCl₄,MoF₅, RuF₅, TaBr₅ and SnI₄; noble metal halides such as for exampleAuCl₃ and HAuCl₄; or organic materials such as for example benzoquinone,tetrachlorobenzoquinone, tetracyanoquinodimethane, ordichlorodicyanobenzoquinone. Specific examples of suitable n-typedopants that can be used in the present invention include, but are notlimited to, alkali metals such as for example Li, Na, K, or Cs, oralkylammonium ions such as for example tetraethylammonium ions ortetrabutylammonium ions.

The CNTs and the dispersant may be mixed in a weight ratio of, but notlimited to, 1:0.005 to 1:100. If the dispersant is used in an amount ofless than the lower limit defined in the range, optimum dispersioneffects of the CNTs cannot be attained. Meanwhile, if the dispersant isused in an amount exceeding the upper limit defined by the range, thereis a possibility that the high conductivity of the CNTs may decrease,causing negative effects.

The transmittance of the CNT thin film can be appropriately determinedby those skilled in the art according to the intended applications andneeds. For example, to use the CNT thin film as a transparent electrode,it is desirable to adjust the visible light transmittance of the CNTthin film to 60% or more and preferably 75% or more at a wavelength of550 nm or 600 nm, at a CNT thin film thickness of about 150 nm.

The CNT transparent electrode using the doped dispersant exhibitsexcellent conductive properties, can be applied to various kinds ofsubstrates, including flexible substrates, and is useful in a wide rangeof applications, for example, as a transparent electrode of a device,such as an image sensor, a solar cell, a liquid crystal display, anorganic electroluminescence (“EL”) display or a touch screen panel.

The transmittance of the structure comprising the CNT thin film andtransparent substrate can also be appropriately determined by thoseskilled in the art according to the intended applications and needs. Forexample, to use the CNT thin film and transparent substrate as atransparent electrode, it is desirable to have a visible lighttransmittance of the structure comprising the CNT thin film andtransparent substrate of 60% or more, preferably 75% or more, at awavelength of 550 nm or 600 nm, at a CNT film thickness of about 150 nmand a transparent substrate thickness of about 150 μm.

In another aspect, the present invention is directed to a method forproducing the transparent CNT electrode. The method comprises (a)preparing a CNT composition for the formation of a CNT thin film, (b)forming a CNT thin film containing a conductive dispersant on a surfaceof the transparent substrate using the CNT composition, and (c) dopingthe CNT thin film formed on the transparent substrate in a dopantsolution to dope the conductive dispersant contained in the CNT thinfilm.

Materials used in the respective steps are as described above. A moredetailed explanation of the respective steps of the method will be givenbelow.

Step (a): Preparation of CNT Composition for the Formation of CNT ThinFilm.

First, a dispersant is dissolved in a dispersion medium selected fromorganic solvents, water, and mixtures thereof to prepare a dispersantsolution. CNTs and the dispersant solution are used to prepare a CNTcomposition.

Dispersion media that can be used to prepare the CNT compositioninclude, but are not limited to, organic solvents, water, mixtures oftwo or more organic solvents, and mixtures of at least one polar organicsolvent (e.g., hydroxyl-containing organic solvents) and water.

Any organic solvent that is commonly used in the art may be used.Examples of suitable organic solvents include alcohols, ketones,glycols, glycol ethers, glycol ether acetates, acetates, and terpineols.These organic solvents can be used alone or in combination.

In an embodiment, the concentration of the dispersant in the dispersantsolution can be in the range of about 0.000025% to about 50% by weightbased on the total weight of dispersant and dispersant medium, and theCNTs are present in the CNT composition an amount of about 0.005% toabout 1% by weight, based on the total weight of CNT and dispersantsolution. Another solvent may be used to dilute the CNTs in the CNTcomposition depending on the mode of formation of a thin film.

Step (b): Formation of CNT Thin Film Containing the ConductiveDispersant on Transparent Substrate Using the CNT Composition.

In this step, the CNT composition prepared in step (a) is used to form aCNT thin film containing the conductive dispersant on a surface of thetransparent substrate.

The formation of a CNT thin film on a surface of the transparentsubstrate may be achieved by depositing the CNT composition by a generalcoating technique, such as spin coating, spray coating, filtration, orbar coating. In an embodiment, a suitable coating technique can beselected depending on the characteristics of the solution and intendedapplications.

The surface of the transparent substrate can, prior to deposition of theCNT composition, be pretreated by a conventional process, such as forexample, but not limited to, O₂ plasma treatment.

The transmittance of the CNT thin film can be appropriately determinedby those skilled in the art according to the intended applications andneeds. For example to use the CNT thin film in a transparent electrode,it is desirable to adjust the visible light transmittance of the CNTthin film to 60% or more and preferably 75% or more at a wavelength of550 nm or 600 nm, at a CNT thin film thickness of about 150 nm

The transmittance of the CNT thin film electrode structure comprisingthe CNT thin film and transparent substrate can be appropriatelydetermined by those skilled in the art according to the intendedapplications and needs. In order to use the CNT thin film andtransparent substrate as a transparent electrode, it is desirable tohave a visible light transmittance of the CNT thin film and transparentsubstrate of 60% or more, preferably 75% or more, at a wavelength of 550nm or 600 nm, at a CNT film thickness of 150 nm and a transparentsubstrate thickness of about 150 μm.

Step (c): Doping of the Conductive Dispersant Contained in the CNT ThinFilm.

In this step, the dispersant contained in the CNT thin film is dopedusing a dopant solution to increase the conductivity of the CNT thinfilm.

The doping of the dispersant can be performed by a chemical process,such as a solution or vapor process.

In an embodiment, in the solution process, step (c) can include thesub-steps of i) doping the transparent substrate having the CNT thinfilm formed thereon in a dopant solution, ii) washing the dopantremaining in the CNT thin film, and iii) drying the doped CNT thin film.

The doping can be achieved by a general doping technique, such asdipping, spin coating or spray. For example, doping of the conductivedispersant by the solution process is achieved by dipping the CNT thinfilm in a dopant solution for a specified time, preferably about 10minutes to about 24 hours, and washing the doped CNT thin film by atechnique known in the art to remove the dopant remaining in the CNTthin film. For example, in an embodiment, the washing is performed bydipping the doped CNT thin film in a nitromethane solution for about 10minutes to about 24 hours.

Thereafter, the washed CNT thin film is dried by a common dryingtechnique to complete the doping of the conductive dispersant. At thistime, the drying is preferably performed at about 80° C. for about 2hours and at room temperature for about 6 hours, sequentially. Thedrying time and temperature can be appropriately adjusted and controlleddepending on the solvent present in the dopant solution.

In another embodiment, in the vapor process, step (c) can be carried outby placing the transparent substrate having the CNT thin film formedthereon in a reactor, filling the reactor with a halogen gas selectedfrom iodine (I₂), bromine (Br₂), chlorine (Cl₂), iodine monochloride(ICl), iodine trichloride (ICl₃) or iodine monobromide (IBr), andexposing the transparent substrate to the halogen gas for about one houror more, but is not limited to this procedure.

Hereinafter, the present invention will be explained in greater detailwith reference to the following examples. However, these examples aregiven for the purpose of illustration and are not to be construed aslimiting the scope of the invention.

EXAMPLES Example 1

i) Formation of CNT Thin Film

20 mg of a conductive dispersant represented by Formula 3 was dissolvedin 20 ml of water, and then 20 mg of purified SWCNTs (single-walledcarbon nanotubes) synthesized by an Arc-discharge process (GradeASP-100F, ILJIN Nanotech Co., Ltd.) was added to the solution.

For a conductive dispersant of Formula (3), Z is selected from the groupconsisting of —H, —CH₃, —OH, carboxylic acid and its salts, sulfonicacid and its salts, and phosphoric acid and its salts, and n is aninteger. For the present Example 1, Z in Formula (3) is a sulfonic acidsodium salt group, to provide the exemplary conductive dispersant, asused herein, having Formula (4).

The conductive dispersant of Formula (4) as used herein had a molecularweight of 10,000 (n=37). The mixture was dispersed in a sonic bath for10 hours, and centrifuged at 10,000 rpm for 10 minutes to prepare a CNTcomposition.

The CNT composition thus prepared was subjected to filtration to form aCNT layer on a transparent polyester film as a transparent substrate,and dried at 60° C. for 2 hours. The visible light transmittance of theresulting structure was measured to be 77% at 600 nm, at a CNT filmthickness of 150 nm and a thickness of the transparent polyestersubstrate of 150 μm.

ii) Doping of CNT Thin Film

The resulting structure was dipped in a 0.25 M nitromethane solution of2,3-dichloro-5,6-dicyano-p-benzoquinone (“DDQ”) as a dopant for 4 hoursand washed in a nitromethane solution for 12 hours to remove the dopantremaining in the CNT thin film. Thereafter, the washed structure wasdried at 80° C. for 2 hours and at room temperature for 6 hourssequentially to produce a transparent CNT electrode.

Example 2

A transparent CNT electrode was produced in the same manner as in stepi) of Example 1, except that a 0.1 M nitromethane solution of goldtrichloride (AuCl₃) was used as the dopant solution.

The sheet resistance was measured for a transparent CNT electrodeproduced in the same manner as in step i) of Example 1 but withoutdoping (i.e., before doping), and for the transparent CNT electrodesproduced in Examples 1 and 2 after doping, and a comparison of theresults are shown in Table 1. The surface of the transparent CNTelectrode produced in Example 1 was observed by scanning electronmicroscopy (SEM) (FIG. 2).

TABLE 1 Transparent CNT Sheet resistance Relative value Electrode^(a)(Ω/sq.) (%) Before doping 817 100 After doping Example 1 394 48.27Example 2 111 13.59 ^(a)The CNT electrodes have a visible lighttransmittance of 77% measured at 600 nm and a CNT thin film thickness of150 nm and a transparent polyester substrate thickness of 150 μm.

Evaluation of Physical Properties of the Transparent Electrodes.

(1) Measurement of Transmittance

The transmittance of the transparent electrode structure consisting ofthe CNT thin film and transparent substrate was measured using aUV-Visible spectrophotometer, at a CNT thin film thickness of 150 nm anda transparent substrate thickness of 150 μm.

(2) Measurement of Sheet Resistance

The sheet resistance (Ω/sq.) of the transparent electrodes wasdetermined using a four-point probe.

As can be seen from the results of Table 1, the sheet resistance valuesof the transparent CNT electrodes measured after doping were reduced byan amount greater than about 50% and less than about 90% when comparedwith those of the transparent CNT electrodes measured before doping.From the results, it can be confirmed that when the transparent CNTelectrodes are used as transparent electrodes of devices, such as imagesensors, solar cells and liquid crystal displays, excellentcharacteristics of conductivity, transparency, and flexibility wheredesired can be imparted to the devices.

Example 3

Transparent CNT electrodes were produced in the same manner as inExample 1, except that CNT thin films were formed on respective PETfilms treated with O₂ plasma by spin coating. The transparent CNTelectrodes thus produced had a visible light transmittance of 79.51% at600 nm at a CNT thin film thickness of 150 nm and a PET film thicknessof 150 μm. The transparent CNT electrodes were used to analyze effectsaccording to the concentration of dopant solutions and the penetrationtime of dopants. At this time, nitromethane solutions having differentconcentrations (0.01 M and 0.1 M) of gold trichloride (AuCl₃) were usedas dopant solutions, dipping was performed for different times (10minutes, 20 minutes, one hour, and 4 hours), and washing was performedby rinsing with a nitromethane solution three times and dipping in therinsing solution for one hour. The washed structures were driedovernight at room temperature. The transmittance and sheet resistance ofthe transparent CNT electrodes were measured by the procedure describedin Example 1, and the results are shown in FIG. 4 and Table 2.

TABLE 2 Relative Sheet resistance Sheet (Rs) value DopingTransmittance^(b) resistance (% of Dopant time (%) (Ω/sq.) reference)Reference X 79.51 2061.15 100 (No doping) 0.01 M AuCl₃ 10 min. 84.76416.76 20 20 min. 83.56 423.56 21 30 min. 83.90 425.82 21 1 hr. 84.96439.41 21 4 hr. 85.38 346.55 17 0.1 M AuCl₃ 10 min. 86.48 265.01 13 20min. 86.31 249.15 12 30 min. 88.18 226.50 11 1 hr. 85.77 194.79 9 4 hr.86.57 131.37 6 ^(b)measured at 600 nm, a CNT thin film thickness of 150nm, and a transparent polyester substrate thickness of 150 μm.

Example 4

Five transparent CNT electrodes with different transmittance values wereproduced in the same manner as in Example 1, except that the contents ofCNTs were varied, a 0.1 M nitromethane solution of gold trichloride(AuCl₃) was used as the dopant solution, and CNT thin films were formedon respective PET films treated with O₂ plasma by spin coating. Thetransparent CNT electrodes were used to analyze the doping effects withvarying transmittance values. The transmittance (% T) and sheetresistance (Rs) of the transparent electrodes were measured by theprocedure described in Example 1, and the results are shown in FIG. 5and Table 3.

TABLE 3 Relative Sheet Resistance (Rs) value (% Sheet resistance ofbefore Sample Transmittance^(c) (Ω/sq.) doping No. (%) Before dopingAfter doping resistance) 1 86.9 1291.62 855.8 66.6 2 85.7 933.59 739.155.2 3 81.6 650.34 536.8 69.7 4 75.8 555.17 311.2 35.0 5 74.5 387.49272.3 77.9 ^(c)measured at 600 nm, a CNT thin film thickness of 150 nmand a transparent polyester substrate thickness of 150 μm.

The sheet resistance values of the transparent CNT electrodes producedaccording to the above method decreased by an average of greater than60% over the entire range of transmittance values, regardless of changesin the transmittance of the transparent electrode structures.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications and variations are possible,without departing from the scope and spirit of the invention asdisclosed in the appended claims. Accordingly, such modifications andvariations are intended to come within the scope of the claims.

As apparent from the above description, according to the transparent CNTelectrode of the present invention, a doped dispersant is used toconsiderably increase the conductivity of a CNT thin film. Accordingly,the transparent CNT electrode of the present invention is useful in awide range of applications, for example, as a transparent electrode of adevice. In addition, the transparent CNT electrode of the presentinvention can be produced in a simple manner by a room temperature wetprocess without involving a sputtering process requiring expensivevacuum equipment, which is efficient in terms of production cost andprocessing, and exhibits excellent characteristics, such as hightransmittance and low resistance.

Furthermore, the transparent electrode of the present invention can beused as a flexible transparent electrode of a flexible display such asthat currently in the spotlight as a next-generation display.

1. A transparent CNT electrode comprising a transparent substrate and a CNT thin film formed on a surface of the transparent substrate wherein the CNT thin film is formed of a CNT composition comprising CNTs and a doped dispersant.
 2. The transparent CNT electrode according to claim 1, wherein the transparent substrate is a transparent inorganic substrate selected from glass or quartz substrates, or a flexible transparent substrate made of a material selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, polyethylene sulfone, polycarbonate, polystyrene, polypropylene, polyester, polyimide, polyetheretherketone, polyetherimide, acrylic resins, olefin-maleimide copolymers, and norbornene resins.
 3. The transparent CNT electrode according to claim 1, wherein the CNTs are selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, rope carbon nanotubes, and combinations thereof.
 4. The transparent CNT electrode according to claim 3, wherein the single-walled carbon nanotubes are metallic carbon nanotubes.
 5. The transparent CNT electrode according to claim 1, wherein the doped dispersant is a conductive dispersant.
 6. The transparent CNT electrode according to claim 5, wherein the conductive dispersant is poly(3,4-ethylenedioxythiophene), a compound having a structure in which a head selected from groups represented by Formula 1 is substituted with one tail or two tails that are the same or different, and selected from groups represented by Formula 2: [Ar—(X)_(a)]_(l)   (Formula 1) wherein Ar is a C₆ aromatic group or a C₄-C₅ heteroaromatic group containing at least one heteroatom selected from sulfur (S), nitrogen (N) and oxygen (O), X is NH— or CH═CH—, a is 0 or 1, and l is an integer from 5 to 60,

wherein Y is selected from the group consisting of substituted and unsubstituted C₁-C₁₀ alkylene groups, substituted and unsubstituted C₁-C₁₀ alkenylene groups, substituted and unsubstituted C₁-C₁₀ alkynylene groups and substituted and unsubstituted C₆-C₂₀ arylalkylene groups, Z is selected from the group consisting of —H, —CH₃, —OH, carboxylic acid and its salts, sulfonic acid and its salts, and phosphoric acid and its salts, b is 0 or 1, m is an integer from 1 to 9, and n is an integer from 0 to
 9. 7. The transparent CNT electrode according to claim 1, wherein the CNTs and the doped dispersant are mixed in a weight ratio of 1:0.005 to 1:100.
 8. The transparent CNT electrode according to claim 1, wherein the doped dispersant is a dispersant doped with a p-type dopant selected from the group consisting of Lewis acids, protic acids, transition metal halides, noble metal halides, or organic materials; or an n-type dopant selected from the group consisting of alkali metals, or alkylammonium ions, including.
 9. The transparent CNT electrode according to claim 8, wherein the Lewis acids include PF₅, AsF₅, SbF₅, ClO₄ ⁻, BF₄ ⁻, BF₃, BCl₃, BBr₃, SO₃, NO₂(SbF₆), NO(SbCl₆), or NO₂(BF₄), the protic acids include H₂SO₄, HClO₄, HNO₃, FSO₃H, or CF₃SO₃H, the transition metal halides include FeCl₃, MoCl₅, WCl₅, SnCl₄, MoF₅, RuF₅, TaBr₅ or SnI₄, the noble metal halides include AuCl₃ or HAuCl₄, the organic materials include benzoquinone, tetrachlorobenzoquinone, tetracyanoquinodimethane or dichlorodicyanobenzoquinone, the alkali metals include Li, Na, K, or Cs, and the alkylammonium ions include tetraethylammonium ions or tetrabutylammonium ions.
 10. The transparent CNT electrode according to claim 1, wherein the CNT thin film has a visible light transmittance of 60% or more at 550 nm or 600 nm measured at a CNT thin film thickness of 150 nm.
 11. A method for producing a transparent CNT electrode, the method comprising: (a) preparing a CNT composition comprising a CNT and conductive dispersant; (b) forming a CNT thin film on a surface of a transparent substrate with the CNT composition; and (c) doping the conductive dispersant contained in the CNT thin film.
 12. The method according to claim 11, wherein step (c) includes the sub-steps of i) doping the CNT thin film formed on the surface of the transparent substrate in a dopant solution, ii) washing the dopant remaining in the CNT thin film, and iii) drying the doped CNT thin film.
 13. The method according to claim 11, wherein the CNT composition comprises CNTs and a dispersant solution containing the dispersant in a dispersion medium selected from organic solvents, water, or mixtures thereof.
 14. The method according to claim 13, wherein the CNTs and the doped conductive dispersant are mixed in a weight ratio of 1:0.005 to 1:100.
 15. The method according to claim 11, wherein the dispersant is poly(3,4-ethylenedioxythiophene), or a compound having a structure in which a head selected from groups represented by Formula 1 is substituted with one tail or two tails that are the same or different, and selected from groups represented by Formula 2: [Ar—(X)_(a)]_(l)   (1) wherein Ar is a C₆ aromatic group or a C₄-C₅ heteroaromatic group containing at least one heteroatom selected from sulfur (S), nitrogen (N) or oxygen (O), X is NH— or CH═CH—, a is 0 or 1, and l is an integer from 5 to 60,

wherein Y is selected from the group consisting of substituted and unsubstituted C₁-C₁₀ alkylene groups, substituted and unsubstituted C₁-C₁₀ alkenylene groups, substituted and unsubstituted C₁-C₁₀ alkynylene groups, and substituted and unsubstituted C₆-C₂₀ arylalkylene groups, Z is selected from the group consisting of —H, —CH₃, —OH, carboxylic acid and its salts, sulfonic acid and its salts, and phosphoric acid and its salts, b is 0 or 1, m is an integer from 1 to 9, and n is an integer from 0 to
 9. 16. The method according to claim 11, wherein the dispersant is doped with a p-type dopant selected from the group consisting of Lewis acids, protic acids, transition metal halides, noble metal halides, and organic materials; or an n-type dopant selected from the group consisting of alkali metals, and alkylammonium ions.
 17. The method according to claim 16, wherein the Lewis acids include PF₅, AsF₅, SbF₅, ClO₄ ⁻, BF₄ ⁻, BF₃, BCl₃, BBr₃, SO₃, NO₂(SbF₆), NO(SbCl₆), or NO₂(BF₄), the protic acids include H₂SO₄, HClO₄, HNO₃, FSO₃H, or CF₃SO₃H, the transition metal halides include FeCl₃, MoCl₅, WCl₅, SnCl₄, MoF₅, RuF₅, TaBr₅ or SnI₄, the noble metal halides include AuCl₃ or HAuCl₄, the organic materials include benzoquinone, tetrachlorobenzoquinone, tetracyanoquinodimethane or dichlorodicyanobenzoquinone, the alkali metals include Li, Na, K, or Cs, and the alkylammonium ions include tetraethylammonium ions or tetrabutylammonium ions.
 18. The method according to claim 11, wherein the CNT thin film is formed by spin coating, spray coating, filtration, or bar coating.
 19. The method according to claim 11, wherein the doping is performed by a vapor phase process with a vapor of iodine, bromine, chlorine, iodine monochloride, iodine trichloride, or iodine monobromide. 